Lightguide fiber is drawn from a solid glass cylinder or preform. One particular method for fabricating such a preform is described in U.S. Pat. No. 4,224,046 to Izawa et al. which issued on Dec. 13, 1977 and has been commonly referred to as the vapor-phase axial deposition (VAD) method.
The VAD technique uses a refractory starting member, which projects into the central portion of a chamber and rotates as it is withdrawn from the chamber along its axis of rotation. A glass raw material is introduced into a high temperature portion of a flame near the tip of a torch producing a glassy particulate or soot which is directed onto the end of the refractory starting member. A substantially cylindrical, boule of soot is formed on the starting member as it is continuously withdrawn from the chamber at a rate equal to the growth rate of the soot upon the boule. The withdrawn soot boule is then subjected to an elevated temperature to consolidate the porous material into a solid, clear cylindrical lightguide preform.
In such a system only a percentage (e.g., 60-70%) of the oxides formed from the precursor material deposits as the soot boule is being formed and withdrawn. Therefore, it is necessary to have an exhaust system to remove the undeposited soot from the chamber.
Known exhaust systems typically use a tubular exhaust pipe that is connected to an exhaust hood or a system common to a filtering system that is exhausted to the atmosphere. Such exhaust techniques result in the clouding of the deposition chamber due to soot depositing on the inside surface thereof owing to the limited "draw" effected by the exhaust pipe in all regions not proximate the entrance of the tube allowing nonuniform gas flow patterns within the chamber. Additionally, substantial fluctuations in exhaust line pressure, originated from in-line blower equipment degrades the stability of the combustion flame at the output of the soot torch. Such systems also have poor reproducibility from day to day due to fluctuations in atmospheric pressure variations which can typically alter precursor delivery from the torch by 15 to 20 percent.
Thus, there is a need for a vapor deposition chamber exhaust system that can remove substantially all of the undeposited soot from the chamber while maintaining a substantially constant pressure and uniform gas flow patterns therein.